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7 Main Evidences Behind Biological Evolution of Man (with diagram)

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Read this article to learn about the Seven Main Evidences Behind Biological Evolution of Man (with diagram) !

The idea of biological evolution probably started with Charles Darwin who defined the term evolution as “descent with modification”. The word ‘descent’ refers to the process of origin of new species from an ancestral stock. Again, the word ‘modification’ introduces an idea of change, which is inherent in evolution.

Therefore, both the words jointly mean a species that evolves from its ancestors through changes. This principle is also applicable to man. Man being the highest product in the evolutionary sequence has descended from non-human ancestors. Some more primitive forms may be stated as ancestors of the ancestors.

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In this way we can reach back to the unicellular form, which formally stand as father of all organisms. To prove this fact, evidences are required for the whole path of biological progress. The biological evolution is exactly same to what we mean by organic evolution.

The organic evolution is concerned with two different types of development, e.g. ontogeny and phylogeny. The ontogeny refers to the history of development of individual organisms. For example, man, like other organisms, began his life as a single cell, which underwent a complex process of development and finally culminated in a multi-cellular adult.

The ontological changes are of great importance to the embryologists. But a student of evolution primarily deals with the second kind of development, which has been referred to as phylogeny. The phylogeny treats with the evolution of a genetically related group of organisms, in contrast to the development of the individual organism. However, the evidences of organic evolution come from different branches of science, like comparative morphology and anatomy, embryology, palaeontology, physiology, biochemistry, genetics, etc.

Seven Evidences have been cited here:

1. Morphological Evidences:

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The structure of animals, both external and internal, provides a source of evidence for evolution. The branch of biology that deals with the form and external structure of animals and plants is called morphology. By comparing organs, musculature and tissues, it can be concluded that man and some other vertebrates have developed from a same stock following evolution.

The general similarities between man and higher vertebrates are so close that the medical students, especially beginners often learn the elements of surgery by desecting the dogs, cats, monkeys and other vertebrates, because bodies of those animals can be procured easily.

Muscles and tissues of man and ape are usually same in number as well as in function, although the forms are slightly different. Morphological peculiarities of man are mainly counted with the erect posture, stability and solidity of the hind limbs (legs), greater prehensility of the forelimbs (arm), and enlargement of the brain, reduction of the face and jaw, and the different type of dentition. In spite of all these facts, no zoologist will hesitate to classify them in the group of primates along with the apes and monkeys, so far as the morphology is concerned.

2. Anatomical Evidences:

The term anatomy is nearly synonymous with morphology and it deals solely with the internal structure of organisms. A comparative study of anatomy reveals the similarities and dissimilarities between man and the other higher primates.

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The overall skeletal structure and bones are more or less same in man and apes. A layman says, ‘man is a descendant from monkeys.’ But a student of evolution never agrees with this idea. According to him, as man and monkey share same common anatomical features, they are contemporaries. Probably both of them arose from a common ancestor (an unknown primate, who is not living today) in the remote past. That common ancestor had the potentialities to give rise to man and monkeys, but no known monkey have that potentiality.

Evolution of the organisms can be better demonstrated with the help of homology and analogy. Homology is the similarity in structure between parts of different organisms due to common origin. Structures having similar genetic basis are said to be homologous. The homologous organs need not be employed for the same purpose.

On the other hand, analogy is the similarity in function between anatomical parts of different structure and origin. These structures usually show superficial similarities due to similar function. Structures having similar functions or projecting similar habits are said to be analogous.

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The forelimb skeleton of vertebrates such as birds, bats, whales, and horses and human are all homologous structures, as these animals possess a similar developmental origin and hereditary basis. But these forelimbs do not look alike as their function differs greatly. In fact, activity of these animals has changed due to environmental influence and the change has been reflected in the adaptability of the forelimbs. Contrary to this, the wings of insects and birds are analogous organs.

Although both are used in flight, they differ in their origin and structure. The wings of insects are composed of chitin (exoskeleton), and are supported by hollow tubes (veins). These wings are therefore non-living structures, operated by certain muscles present at their bases. Conversely the wings of birds are supported by a living endoskeleton which remains covered externally by feathers.

The supporting endoskeleton has several segments namely humerus, radius and ulna, carpo-metacarpus and the phalanges. Similar analogous structures are found in the fins of fishes and the flippers of the aquatic mammal, whale. Both serve the function of swimming but differ in their skeletal arrangements. Analogical organs may also develop for the purpose of adaptation. Different groups of animals when face same type of environment, they work out more or less similar type of devices for survival.

The vertebrates like fishes, birds, amphibians and mammals are found to possess more or less the same organs in their body e.g. heart, livers, urinary system, nervous system, etc. Now, if we consider any one of these organs, say for example, the heart, we will be able to find out a gradual change in the structure of heart which occurs with time. The structure gets gradually complex in higher forms. For instance, a fish has a two-chambered heart. Again, the mammals being developed in form bear a four-chambered heart. Thus comparative anatomy between the animals yields various evidences of evolution.

3. Vestigial Evidences:

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The word vestige means ‘a small trace’. In any living organism, the vestigial body parts serve no remarkable purpose; they lie as rudimentary organs or structural residue in living body. Functional analysis of these organs is apparently irrelevant as they show little or no use. But the same functional analysis stands significant to understand the emergence of a new species by evolutionary modification from earlier biological forms. They are able to throw considerable light on the structure of more primitive forms.

Therefore a vestige can be an important criterion in tracing organic development. An example may be cited with appendix which serves a definite digestive function in apes, monkeys and other herbivorous animals. In man the vermiform appendix is not only a useless structure, but also a seat of a dangerous disease called appendicitis. The appendix remains attached to a short portion of the large intestine, termed as caecum. Both the appendix and the caecum are fairly well developed in herbivorous mammals, whose diet consists of abundant cellulose.

In these mammals, the appendix and caecum form storage compartments, in which, the mixtures of partly digested food and enzymes stay for a considerable length of time. At this time bacteria act on the cellulose to transform it into easily digestible chemical compounds. Man might have inherited these vestigial structures from his remote ancestors whose diet contained a considerable amount of cellulose.

Another vestigial structure occurs in the inner angle of each eye of man. It appears like a little fold of flesh and is termed as semilunar fold (Plica semilunaris). This structure represents the reduced, movable, third eye-lid, and called nictitating membrane in lower vertebrates, where it is used to clean the eyeball. But no implication is found in the body mechanism of man.

The presence of ear muscles in man is another instance of vestigial structure. In several mammals the external ears are fully movable to hear the sounds coming from several directions. The rudimentary muscles present in the skin of ear of man are incapable of moving the ears; although some individuals have the ability to ‘wriggle’ their ears.

Again, the third molar tooth, which is functional as well as useful for the primates, shows no use in man. It is called “wisdom tooth’ which hardly finds its place in the reduced jaw of man. Usually it erupts much later and sometimes it fails to erupt, as its requirement is minimal in human dentition.

Similar example may be drawn with some specific parts of human skeleton. The spines of the four vertebrae at the lower end of vertebral column fuse together to develop a small round bony structure, known as coccyx or tailbone. This hidden vestigial tailbone indicates that man had tail in certain phases of his development; some of his ancestors definitely possessed this tail.

The hidden tail is also seen in the human embryo during its second month of growth. Sometimes a few babies are born with an external coccyx, which has to be corrected surgically by medical practitioners. The presence and distribution of body hair in man may be regarded as vestigial. Superfluity of hair IS found among the anthropoid apes as well as in the pre-hominid forms.

Dr. Weidersheim gave a list of about hundred vestigial structures in man alone. Such organs are also found in every specialized animal in its anatomy. For example, though snakes are usually limbless in character, vestiges of hind limbs are seen in pythons and boas. In these snakes hind limbs are represented by reduced pelvic girdle, capped by claws.

In horse, only the third digit is present- the metacarpal of digit IH has greatly enlarged to form the cannon bone. Vestigial structures in the leg of horse are found as splint bones, which get fused with the sides of cannon bone. Similarly the vestigial organs are found among the birds. Birds are notable for their flying habits, but a few of them are flightless.

For instance, in Kiwi (a bird of New Zealand), the wings are vestigial which remain concealed by the feathers of the body. Again, the animals, which live in perpetual darkness like deep caves, generally show degenerative changes in their eyes. The cave-dwelling salamander of Central Europe is characterized by complete absence of eyes. Several cave-dwelling fishes are also blind, as their eyes are functionless.

Different vestigial organs are thus formed in the course of evolution due to the loss of their typical function. Consequently these organs show a great reduction in size and as the evolutionary changes continue, they get a chance to be eliminated completely from their possessors.

4. Embryological Evidences:

Embryology is a specialized branch of biology, which deals with the formation and development of the embryo. Comparative embryology furnishes several outstanding evidences for evolution in 1866, the German scientist Ernst Haeckel (1834 — 1919) formulated the ‘Biogenetic Law’ which is more often known as the ‘Theory of Recapitulation’.

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The theory states that ‘Ontogeny Recapitulates Phylogeny’ which means, the embryos in their development repeat the adult stages of their ancestral forms. The fundamental concept of recapituation stems from Karl Ernst Von Baer (1792 — 1876) who stated “The embryo of a higher species may resemble the embryo of a lower species but never resembles the adult form of that species”.

The human embryonic development furnishes several evidences of recapitulation. Each human being starts his life as a single cell, the fertilized egg or zygote, which corresponds to a protozoan ancestor. The zygote divides and becomes a multicellular blastula, which may be comparable to a Colonial flagellate like volvox.

The blastula undergoes gastrulation to form a two-layered coelenterate like embryo, which eventually transforms into a triploblastic structure resembling a flatworm. Then chordate features (notochord, dorsal tubular nerve cord, and pharynx specialized for respiration) appear being followed by the development of gill silts and aortic arches like those of fishes.

Next, tetrapoid characters like pentadactyl limb and metanephric kidney make their appearance. This is followed by the development of mammalian, then primate, and finally specific human characters. But, the fact does not mean that the embryonic stage of a fish and human is quite identical. The presence of gill arches in human embryo points out that man’s ancestor to distant past had the evidence of gill. Thus embryology serves as a source of evidence in connection with evolution.

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Thus embryos of different animals remain similar at an early stage, including the man. Differentiation among them is possible only at a later stage of embryonic development.

5. Palaeontological Evidences:

Palaeontology is the study of fossil remains, which provides very reliable information regarding the fauna of particular periods. Any type of remain of living form from the remote past as obtained from the earth may be considered as a fossil. The term fossilization implies the process of fossilizing. Fossil may take any form, and the nature of the fossil record is varied, which may range from an intact mammoth to a simple footprint of a dinosaur.

Any organism may undergo fossilization, but in reality, only a few of them become fossilized. The majority of organisms disintegrate before being preserved for fossilization. Two general types of fossil record have been observed —unaltered remains and altered remains.

The unaltered fossils are very rare in nature. The woolly mammoths of Siberia that are preserved in the permanently frozen tundra constitute the examples of this type. Similarly, insects encased in fossil resin (popularly called amber) in the beds of Baltic Shores of Prussia are also the examples of unaltered remains.

In altered remains the fossil show great variations in their constitution, and in most instances the original organic parts become gradually replaced by inorganic salts like calcium carbonate, silica, iron pyrite etc. The fossils may take the form of mould (external impression) or natural casts, or it may be in the form of trails, tracks, footprints, leafprints, burrows, borings, tubes, etc.

A brief evaluation of some of the fossil records, especially pertaining to fossil men and pre-hominoid forms present fascinating evidences in support of human evolution. It should be remembered that the Protochordata have not left any fossil record, as they have no bony skeleton. Evolution of horse can be traced beautiful y because a complete sequence of fossil horses has been recovered from different geological strata, n case of man, fragments and bones are the mute testimony for the antiquity of man and his early nature.

6. Evidences from Physiology and Biochemistry:

Physiology is the science that studies the activities of organisms. Biochemistry, on the other hand is concerned with the chemical structure and processes occurring in organisms. In some cases, evidences of physiology and biochemistry have been proved useful in tracing the relationships among certain organisms, which could not be drawn on morphological basis.

Similarities of chemical structure and function are so fundamental that these are essentially similar in large groups of animals and plants. It has been estimated that there are slightly more than hundred chemical elements. All living organisms, plants as well as animals are composed of one or more of these elements, which remain combined in appropriate proportions.

Four of them Carbon, Hydrogen, Oxygen and Nitrogen are the most common elements; 99 percent of living things including amoeba, euglena pea-plant and man are formed in combination of these elements. Again, the abundantly present chemical elements mostly constitute three main organic compounds viz., Carbohydrates, fats and proteins.

Carbohydrates such as starches, sugars, cellulose etc. are composed of carbon hydrogen and oxygen. Fats are similarly constituted of carbon, hydrogen and oxygen. Nitrogen and phosphorus are there, in addition. Protein is composed of carbon, hydrogen, oxygen, nitrogen, sulphur and phosphorus.

These three organic compounds combine together in varying ways to form a complex substance called protoplasm. Huxley considered it as ‘the physical basis of life’. The protoplasm of all living things-from amoeba to man, has several similar physical and chemical properties. This protoplasm is built into structural units of living organisms called cells. The cells being ‘building blocks’ of animals and plants exhibit fundamental similarity in all living things.

The study of cell physiology has revealed that the process of cell division (mitosis) is essentially similar throughout the plain and animal kingdom. Besides, a large group of animals share some common chemical structure and function of enzymes and hormones in their physiology.

The proteolytic enzyme, trypsin, which takes part in protein metabolism, occurs in several groups of animals from Protozoa to the Mammalia. Similarly, the starch-splitting enzyme, amylase, is found in animals ranging from the Porifera to the Mammalia.

The serology deals with blood serums, and their reactions and properties. Serological tests provide very impressive physiological evidences concerning with evolution. The specialized bio-chemical study, immunology (based on antigen-antibody reaction) reveals the fact that the reaction between antiserum and blood serum of man follow the same course as found in the apes.

These serological tests not only confirm the relationship of man to other pronates (particularly with anthropoids like chimpanzee), which were originally based upon comparative morphology, but also establish the degree of similarity of the serum proteins among the related species. The comparative serology also helps in resolving difficult taxonomical problems where standard morphological evidences fail to verify the taxonomy of certain aberrant animals.

7. Evidences from Genetics:

Genetics is an important branch of biology, dealing with the science of heredity and variation. The principal fields of genetics from which evidences for evolution are drawn include hybridization, domesticated animal and plant species, gene and chromosome homology, and the nature of genetic (hereditary) material.

The hybridization involves crossing (cross mating) of two genetically unidentical individuals (usually two species) which leads to the production of hybrid progeny. One of the most fascinating examples of animal hybrid is the mule, which is a hybrid species, resulting from a cross between an ass and a horse.

The mule combines the hardiness of the ass and the sensibility of the horse. But this species is considered as evolutionary dead because the mules are invariably sterile. Such instances are very rare in nature and usually two clearly distinct species on hybridization produce viable and vigorous progeny. The sterility among the offspring individuals is due to genetic incompatibility. Therefore, the genetic study can reveal the closeness between the animal forms in connection to evolutionary consequences.

Domesticated species of animals and plants are of considerable interest to the students of evolution as they provide magnified view of evolution, although in a somewhat distorted manner. In fact, the domesticated animals of today are descendants from wild progenitors.

The innumerable breeds or varieties of domesticated species have arisen due to artificial selection by man, but the natural selection also operates side by side. However, the evolutionary significance of domesticated species lies in the fact that they often exhibit considerable changes from their wild counterpart.

Relationships between species can be determined genetically by comparing their chromosomes. The genetic experiments have revealed that the unpaired parts of the chromosomes differ in their genetic contents. The similarities in genetic behaviour between homologous chromosome regions signify the degree of relationship of related species. This is the most reliable evidence in tracing the line of evolution.

The chemistry of chromosomes, from bacteria to man has revealed that they are composed of nucleoproteins, a combination of nucleic acids and proteins. Among the two nucleic acids, DNA and RNA, DNA is the genetic material in most of the organisms.

Only in certain viruses, RNA is the genetic material. The available evidences indicate that the nature of nucleic acids is fundamentally same in all organisms—viruses, bacteria, plants and animals. Chemically, both DNA and RNA are very similar, in having sugar-phosphate backbone, and nitrogenous bases, purines and pyrimidines which remain attached to sugar molecules as side chains. The differences between DNA and RNA molecules lie in the sugars, and in one of the pyrimidines. In DNA, the sugar is deoxyribose, while it is ribose in RNA. In DNA, the four nitrogenous bases are adenine and guanine (purine), and cytosine and thymine (pyrimidme). In RNA, the thymine is replaced by uracil.

In most organisms (except some RNA viruses), DNA is the genetic material which transmits the hereditary blue print from one generation to the next; RNA appears to take part in protein synthesis. DNA, as a primary hereditary material is capable of undergoing a change, due to genetic phenomenon called mutation. The trait developed as a result of mutation is different from the original, but it is as stable as the original trait. Therefore, mutations account for the hereditary variations, and also form the raw materials of evolution.